A value document refers for the purposes of the invention to bank notes, but also shares, deeds, postage stamps, checks, admission tickets, travel tickets, air tickets, identity cards, visa stickers and the like, as well as labels, seals, packages, security paper or other elements for product protection. Therefore, the simplifying designation “value document” will hereinafter always include documents of the stated kind.
It is known that bank notes, for example, have magnetic material which is used for checking the authenticity of the bank notes. Such magnetic material is for example a component of printing inks and applied to the bank notes upon printing thereof. For checking the authenticity of the bank notes, the distribution of the magnetic material produced upon printing can be determined and compared to a specification. Further, it is known that magnetic material can be contained in security elements applied to the bank note. For example, security threads can have magnetic material which can be applied either continuously or in the form of a coding.
Further, it is known to embed security threads in bank-note paper partially or completely. A partially embedded security thread is visible only in certain places on the surface of the bank note, for example in periodically disposed windows in the bank-note paper. Upon viewing in reflected light, such a (continuously metallized) windowed security thread appears as a periodically visible metallization strip extending e.g. perpendicular to the longitudinal direction of the bank note.
For imitating a windowed security thread, forgers affix for example metallized film strips to forged bank notes which resemble a windowed security thread when viewed in reflected light. It has turned out that such forgeries cause a certain (inductive) measuring signal upon examination with magnetic sensors, although the metallized film strip has no magnetic material. On the other hand, authentic bank notes that are in a poor state can also cause a similar measuring signal. Hence, upon checking the bank notes there is a risk of bank notes that are authentic but in a poor state being incorrectly considered to be forgeries with thus imitated security threads and being mistakenly sorted out.